The history of non-rotary wing V/STOL (vertical/short takeoff and landing) aircraft development has generally proceeded along two separate paths. STOL (short takeoff and landing) aircraft development has centered on conventional aircraft operation, using enhanced lifting devices to shorten takeoff and landing runs. VTOL aircraft development has centered on the use of powerful jet engines to provide the required vertical thrust, and as such has focused on military applications. Latest developments, such as the Osprey aircraft being procured by the US Marines, use two widely spaced proprotors, similar to large propellers, but without shrouds or ducts. These proprotors rotate during flight approximately 90 degrees, from a horizontal to a vertical position to transition from vertical lift to horizontal thrust. The proprotors are too large in diameter for the plane of the rotors to be rotated to a vertical position when sitting on the ground.
Rotary wing aircraft, such as helicopters, have had more commercial success than the military designs, but still rely predominately on turbojet engines to provide the power for vertical lift. Their advantage in vertical lift capability is offset by their poor flight stability characteristics, and very high initial and maintenance costs.
STOL Designs
STOL designs reduce the takeoff and landing runs for aircraft by primarily reducing the stall speed of the aircraft. This is accomplished by increasing either the available wing lifting area or increasing the lift coefficient the wing is capable of producing, by means well known to practitioners of the art. Regardless of the design. STOL aircraft must provide forward movement of the aircraft in order to produce lift, with the notable exception of the Custer channelwing aircraft, which could lift vertically while tethered.
STOL aircraft have the potential to be particularly safe aircraft, since there is a direct relationship between the severity of injuries sustained in crashes and the speed at impact. With a lower potential impact speed due to lower stall speeds, STOL aircraft can be designed to provide complete protection from injury in most crash landing situations.
VTOL Designs
Helicopters
Helicopter designs fall into two broad categories; a single lift rotor with a tail rotor to control yaw, and two lift rotors rotating in opposite directions to control yaw. The single lift rotor is much more popular since it is simpler, but it yields roughly 30% less direct vertical lift for a given horsepower than the dual rotor system. Typically, helicopter rotor systems provide a lifting capability of 10–15 pounds per horsepower, with top speeds in horizontal flight of less than 200 miles per hour.
The flight and maintenance problems associated with helicopters are well known and bear no repeating. Their disadvantages are detailed below in the section on Disadvantages of the Prior Art.
Autogyros and Gyrocopters
Autogyros provide for lift using an unpowered overhead rotor system similar to the helicopter, with auxiliary wings to provide supplemental lift. Gyrocopters do not have auxiliary wings. Most designs provide pusher propellers as the primary means of propulsion. The Cartercopter is a recent example.
Current gyrocopters provide prerotors to spin up the main rotor system to high speed, and then use a “jump” takeoff to lift almost vertically. The gyroscopic inertia of the main rotor is used to lift the aircraft into the air, where it then accelerates forward using the pusher propeller. Once moving forward, the rotor speed is maintained by contact with the moving air. Gyrocopters with prerotors do not have hovering capability in a loitering mode.
Tilt Rotor (Proprotor) Aircraft
This type of VTOL aircraft provides two or more very large propellers mounted on wingtips or wing pylons. The propellers rotate through over 90 degrees of angle, from a horizontal axis for forward flight, through to a vertical axis for vertical lift thrust. Due to the need for redundancy in case of a single engine failure, complex crossover shafting is required. Current aircraft of this type are military, with smaller civilian versions in the planning stages. These aircraft have lower thrust output for a given horsepower than helicopters, but have the capability to exceed 300 miles per hours in horizontal flight.
Direct Lift Aircraft
Ducted Fans
VTOL aircraft using ducted fans have been the most commonly developed prototype, but with little success. While the ducted fan provides greater lift than a simple unducted propeller, the drag produced by the duct shroud in forward flight at speeds over 150 mph has greatly limited the success of this type of VTOL aircraft.
One of the best examples of the ducted fan type of VTOL aircraft is described in U.S. Pat. No. 5,454,531. This aircraft consists of three ducted propeller assemblies consisting of one primary and two control propellers mounted in a triangular fashion. The assemblies are canted with respect to the aircraft fuselage by 45°. For vertical lift, the airstream is deflected an additional 45° downward to produce vertical thrust, and then once airborne, the deflectors rotate the airstream through 90° to the rear to produce forward thrust.
This aircraft has a number of disadvantages, which are discussed, in the following section entitled, “Disadvantages of the Prior Art”.
There are numerous fan-in-wing configurations, which have been patented, but none has proved to be successful. The primary problem has been the high required disc loading causing high exhaust velocities. A good example is illustrated in U.S. Pat. No. 3.083,935, Piasecki, where fans are located in each wing, with the wings having a retracting feature to cover the fans in forward flight.
Deflected Thrust
The greatest military success in VTOL aircraft has come with this alternate, in the form of the Marine Corps Harrier. Other aircraft, including the Moller SkyCar and Soltrek XFV, are in flight testing. The Skycar uses eight separate engines in four deflected thrust lift pods to provide both vertical and horizontal thrust.
Many of the VTOL designs have the capability of STOL operation. In particular, the Harrier is used by the British with a jump takeoff ramp on aircraft carriers to enhance range and payload.